1. Field of the Invention
The present invention relates generally to document processing apparatus and, more particularly, to a document processing apparatus such as an electronic typewriter device arranged to store, print and display input characters.
2. Description of the Prior Art
In general, such a document processing apparatus is designed to formulate a document, and is also capable of formulating a numerical expression by a combination of various kinds of characters such as alphanumerical characters and symbols.
For the purpose of illustration, it will be considered how a conventional type of document processing apparatus is used to formulate the following numerical expression: ##EQU1## In this numerical expression (1), "l", "i", "m", "S", "n" and "a" constitute a group of alphabetical characters, while ".SIGMA.", ".infin.", ".fwdarw.", and "=" constitute a group of symbols. (The following description is made aside from considerations as to the size of each character in a phrase, a sentence or a paragraph.) As is evident from the above example, almost all numerical expressions are represented by a combination of alphabetical characters and symbols or numerical characters.
Normally, a type wheel of a fixed pitch or a proportional pitch (hereinafter referred to as "PS pitch") can be used to print the numerical expression (1).
However, several problems are encountered when numerical expressions need to be readably printed.
By way of example, the following numerical expression will be considered: EQU sin 3x=3 sinx-4 sinx (2)
If a type wheel of a fixed pitch (for example, 10 pitch) is employed for printing, the expression (2) will be printed as shown in, for example, FIG. 14.
The printing shown in FIG. 14 will convey the impression that the expression (2) as a whole has been expanded between the leftmost character and the rightmost character since all the characters are widely spaced apart from one another. This is because, although the "sin" in the expression (2) originally constitutes one meaning by a combination of three characters, the characters "s", "i" and "n" as well as the remaining characters are printed with a substantially equal character spacing. For this reason, the impression conveyed by the individual characters "s", "i" and "n" is not associated by the reader with the true meaning of "sin" in the numerical expression. As a result, he may experience a sense of lack of understanding with respect to the whole of the printed numerical expression (2). In this case, it would be preferable for "s", "i" and "n", which mean "sine", and "c", "o" and "s" which mean "cosine" (although not contained in the numerical expression (2)) to be respectively printed with the spacing between each adjacent character being condensed.
The following is a description, referring to FIG. 15, of a case where the above noted numerical expression (2) is printed by means of a type wheel of a proportional pitch (PS pitch).
In the case of PS pitch, the numerical expression (2) is printed as shown in, for example, FIG. 15. The characters "s", "i" and "n" shown in FIG. 15 are closer to one another, and hence more visible, than those printed at a fixed pitch as shown in FIG. 14.
However, since the spacing between each adjacent character is narrow with respect to all the numerical characters, symbols and alphabetical characters, the numerical expression as a whole seems to be condensed.
When the previously-noted numerical expression (1) consisting of symbols and alphabetical characters is to be printed with PS pitch, it is condensed as shown in FIG. 16. As can be seen from FIG. 16, an imbalance occurs in the relative size of adjacent characters since "a" is extremely small in size compared with ".SIGMA." which appears immediately before it.
In addition, symbols are in most cases printed using a type wheel of a fixed pitch. Accordingly, if printing is effected by a combination of a PS-pitch type wheel and a fixed-pitch type wheel, character spacings are often excessively widened or narrowed.